Technical Field
The disclosure is related to processes and apparatus for forming conductive films on a workpiece such as a semiconductor wafer, and in particular for measuring and controlling uniformity in such films.
Background Discussion
Semiconductor integrated circuits have conductive structures defined by trenches filled with a metal such as copper deposited on a barrier layer formed on or over the semiconductor or dielectric substrate. Copper migration into the semiconductor or dielectric substrate is prevented by forming the barrier layer of a suitable material such as tantalum nitride or titanium nitride, for example. Prior to filling the trench with copper, a thin copper seed layer is grown on the barrier layer by physical vapor deposition. Thereafter, a thick copper layer is deposited over the copper seed layer by electroplating, the thick copper layer being sufficient to fill the trench.
A final overall thickness distribution measurement is required upon completion of the electroplating process. Typically, such a measurement requires an expensive measurement tool, such as an opaque film metrology device employing optical measurements of elastic deformations stimulated by ultrasonic energy. A less costly alternative, such as an eddy current sensor, involves a slower acquisition of thickness distribution requiring discrete measurements at many locations across the wafer surface, using a move-stop-acquire-move sequence. In certain Cu plating processes, there are process failures that exhibit small-area (<10 mm×10 mm) thickness non-uniformity anywhere near the edge of a 300 mm wafer. Detecting such failures requires a high density of measurement locations (>100 sites) along the perimeter of the wafer, decreasing throughput significantly.